1. Field of the Invention
This invention relates to a sheet feeding device for use in a printer in which a continuous printing sheet such as a fanfold paper (hereinafter referred to as "a continuous sheet") and a single printing sheet such as a cut paper (hereinafter referred to as "a single sheet") are fed for printing operation.
2. Prior Art
Some of the printers, which are output devices of a computer, are able to use both a continuous sheet such as a fan-fold paper and a single sheet such as a cut paper. In such a printer, the fan-fold paper is fed with a push tractor disposed in front of a platen, and the cut paper is fed by rotation of the platen itself.
In order to effectively feed the papers, Japanese Patent Application (OPI) No. 145052/1985 has disclosed a printer which has two holding rollers below the platen (the term "OPI" as used herein means an "unexamined published application"). In the printer, a fan-fold paper feeding mode and a cut paper feeding mode are switched over to each other according to the kind of printing paper to be used. That is, in the fan-fold paper feeding mode, the roller further from the printing head is abutted against the platen to lead the paper, and then the two rollers are moved away from the platen. In the cut paper feeding mode, only the roller closer to the printing head is abutted against the platen.
The above-described conventional printer is advantageous in that, since it is so designed that the roller further from the printing head is abutted against the platen, the printing sheet can be readily loaded. However, it is still disadvantageous in the following points: Even if, as shown in FIG. 1 (Prior Art), the front end portion of the printing sheet s is fed to the platen c with the aid of the roller a and a guide board b, in order to let it go along the platen c it is necessary to provide an auxiliary sheet guide board d so as to forcibly lay the printing sheet over the platen. In this case, the printing sheet, striking against the auxiliary sheet guide board d, is held unstable; i.e., slackened between the roller a and the auxiliary sheet guide board d. Therefore, in a printer in which the peripheral speed of the platen c is higher than the sheet feeding speed of the push tractor e, the slackening of the printing sheet is gradually absorbed during the sheet feeding operation, as a result of which the print position is shifted as much as the slackening. Furthermore, in the cut paper feeding operation, the cut paper cannot be sufficiently fed with only the roller closer to the printing head.
A sheet feeding device in a printer, which can use both a continuous sheet and a single sheet by switching the continuous sheet feeding mode and the single sheet feeding mode over to each other as was described above, has been also disclosed by Japanese Patent Application (OPI) No. 205182/1982. In the device, in order to use a single sheet, its change-over lever is shifted to interrupt the transmission of the torque of an electric motor to the pin tractor, so that the single sheet can be fed with a pair of drawing rollers without removal of the continuous sheet from the pin tractor. That is, in the device, the continuous sheet feeding mode and the single sheet feeding mode are switched over to each other by operating the change-over lever only. Therefore, the sheet feeding device is unavoidably intricate in construction. Furthermore, the operation of the device is rather troublesome because the change-over lever must be operated frequently.